Electroless plating of optical fibers for hermetic feedthrough seals

This paper describes an approach to plating solderable Ni-P metal on a fiber and sealing it into a Kovar ferrule. We demonstrate long-term reliability of the resulting soldered joint by detailed measurement of the fiber and joint strength before and after accelerated aging. We present evidence that the initial weakening of the fiber is caused by the plated metal itself, that failure of the metal film initiates fracture of the fiber. To support this, we show that embrittling the metal by heat treatment further reduces the strength of the metallized fiber. Since we intended to use the process on polarization-maintaining fibers, which are sensitive to stresses, we used a soft solder to make the hermetic seal. The joint strength is limited by the shear strength of the soft solder; use of a Au/Sn eutectic (m.p. of 280°C) probably results in embrittlement of the Ni-P and limited increase in strength